Invert normals only for selected polygons

is there a way to flip all the face normals for selected polygons in one direction, especially in python?
The intended purpose is to fix a completly "broken" mesh to correct the background culling for Unity.

Thanks in advance!

Julien

Example:
All stair polygons should face upwards.
Tried align normals and optimize commands, but they did not work with this mesh.

Finally, I would like to point you to https://code.vonc.fr/?a=23 which offer a selection based on the normal direction. So you may be able to select all downward normals and call inverse Normal command.